The present invention relates to axial entry dovetail connections between a turbine wheel and turbine buckets and particularly relates to a complementary anti-rotation key and recess on the axial entry bucket and wheel dovetails, respectively, to minimize or eliminate rotation of the bucket dovetail in the wheel slot.
For axial entry turbine bucket-to-wheel connections, each bucket typically includes a male dovetail extending in an axial direction and having a plurality of hooks along opposite sides. Female dovetail slots are provided at circumferentially spaced locations about the margin of the turbine wheel and receive the male dovetails of the buckets in an axial direction to secure the buckets and wheel to one another. Pins or other devices are conventionally used to finally secure the buckets and wheel to one another. Oftentimes, each bucket includes a bucket cover and certain turbine buckets have a steep angle integral cover. A steep angle bucket cover has lateral edges facing in a circumferential direction for mating with correspondingly-shaped edges of adjacent covers. By steep angle is meant that the lateral edges have a steep angle relative to the direction of rotation of the bucket, i.e., forms an acute angle with a tangent of the bucket. In bucket covers having a generally Z-shaped configuration along their opposite edges, the intermediate portion of the Z-shaped edge manifests this steep angle relative to a tangent. When buckets of this type having steep angle integral covers are assembled onto the wheel, second and subsequent buckets being installed cause an interfering contact between the steep angle edges of the bucket cover being installed and the previously installed adjacent bucket cover. This interference prevents the bucket from full axial insertion and, accordingly, a force is typically applied to complete axial insertion of the bucket dovetail into the wheel dovetail. This force tends to twist or apply a torque to the bucket along its outer periphery, which twist is transmitted through the airfoil to the bucket dovetail. This transmitted twisting or torque causes the male dovetail on the bucket to engage along the crush surfaces of the female dovetail, causing both a radial inward movement of the bucket, as well as a twisting motion to the bucket. The radial inward movement causes an overall reduction in the diameter of the bucket covers upon completing assembly of the buckets onto the wheel. This causes a problem when the bucket covers are machined to the designed diameters subsequent to bucket installation. When the turbine comes up to speed in operation, the buckets will, under centrifugal forces, move radially outwardly so that the bucket dovetail hooks seat on the crush surfaces of the wheel dovetails. The bucket covers will thus ride closer to the spill strips and may cause rubs. Also, because of this twisting action upon assembly, the buckets may not lie on a radial line and can easily pivot about the dovetail in a tangential direction. If machined off the radial line, then when the turbine comes up to speed, the buckets will seat themselves on the radial line, causing steps to appear between the bucket covers.
One method of enabling machining of the covers to the correct diameter upon installation of the buckets on the wheel is to insert metal shims under the bottom of each dovetail during assembly. This maintains the buckets radially outwardly in engagement with the crush surfaces of the female dovetails. Thus, while the diameter of the bucket covers can be machined properly using shims, the shims must be removed before shipping the rotor and their removal also requires removal of the buckets, which is a time-consuming task. Accordingly, there is a need for a mechanism to maintain the buckets from rotating on the crush surfaces upon installation of the buckets and on a radial line during cover machining.
In accordance with a preferred embodiment of the present invention, there is provided turbine buckets having axial entry male dovetails and steep angle integral covers wherein the bucket dovetails include an anti-rotation key for minimizing or eliminating any tendency of the bucket dovetail to rotate in the wheel slot in response to interference fits between adjacent covers, enabling machining of the cover profiles to the designed diameters. Particularly, an anti-rotation key and a corresponding recess are provided at the bases of the each male dovetail and female dovetail slot, respectively, which minimizes or eliminates tendency of the buckets to rotate due to an interference between the adjacent covers upon installation of the buckets. The key at the base of each male dovetail comprises a radial inwardly extending projection or lug, preferably extending the axial full length between end faces of the bucket dovetail. Correspondingly, the recess in each female dovetail-shaped slot extends preferably between opposite end faces of the wheel. By providing tight tolerances between opposite sides of the lug and side walls of the recess, any tendency of the bucket dovetail to rotate in the female slot upon axial insertion is substantially eliminated. Thus, radial inward and off-radial line movements of the bucket are precluded upon installation of the buckets.
In a preferred embodiment according to the present invention, there is provided a rotor wheel assembly comprising a rotor wheel having a plurality of circumferentially spaced, axial entry, female dovetail-shaped slots about a margin of the wheel, a plurality of buckets, each having an axial entry male-shaped dovetail generally complementary to the female dovetail-shaped slot for reception in the female dovetail-shaped slot, each slot including a base having a recess radially inwardly of the base and opening through at least one end face of the wheel and radially outwardly into the female dovetail-shaped slot, each male-shaped dovetail having a key projecting radially inwardly from a base of the male-shaped dovetail and received in the recess, the key and walls defining the recess in the base being cooperable to preclude rotation of the bucket generally about a radius of the wheel passing through the bucket.
In a further preferred embodiment according to the present invention, there is provided a rotor wheel assembly comprising a rotor wheel having a plurality of circumferentially spaced, axial entry, female dovetail-shaped slots about a margin of the wheel, a plurality of buckets, each having an axial entry male-shaped dovetail generally complementary to the female dovetail-shaped slot for reception in the female dovetail-shaped slot, each slot including axially extending opposite side walls having flat parallel surfaces opening through at least one end face of the wheel, each male-shaped dovetail having axially extending opposite side walls having flat parallel surfaces, the flat surfaces of the male-shaped dovetail and the flat surfaces in the slot being cooperable to preclude rotation of the bucket generally about a radius of the wheel passing through the bucket.